Editing Geography Markup Language (section)

===Coordinate reference system===
{{main|coordinate reference system}}
A '''coordinate reference system''' (CRS) determines the geometry of each geometry element in a GML document.

Unlike [[Keyhole Markup Language|KML]] or [[GeoRSS]], GML does not default to a coordinate system when none is provided. Instead, the desired coordinate system must be specified explicitly with a CRS. The elements whose coordinates are interpreted with respect to such a CRS include the following:
* {{code|<gml:coordinates>}}
* {{code|<gml:pos>}}
* {{code|<gml:posList>}}

An ''srsName'' attribute attached to a geometry object specifies the object's CRS, as shown in the following example:

<syntaxhighlight lang="xml">
 <gml:Point gml:id="p1" srsName="#srs36">
     <gml:coordinates>100,200</gml:coordinates>
 </gml:Point>
</syntaxhighlight>

The value of the ''srsName'' attribute is a [[Uniform Resource Identifier]] (URI). It refers to a definition of the CRS that is used to interpret the coordinates in the geometry. The CRS definition may be in a document (i.e. a ''flat file'') or in an online web service.  Values of EPSG codes can be resolved by using the [[EPSG Geodetic Parameter Dataset]] registry operated by the Oil and Gas Producers Association at [http://www.epsg-registry.org ] {{Webarchive|url=https://web.archive.org/web/20200809231738/http://www.epsg-registry.org/ |date=2020-08-09 }}.

The ''srsName'' URI may also be a [[Uniform Resource Name]] (URN) for referencing a common CRS definition. The OGC has developed a URN structure and a set specific URNs to encode some common CRS. A URN resolver resolves those URNs to GML CRS definitions.